AN EXTENDED SHALLOW FLOW THEORY FOR NATURAL DEBRIS FLOWS

Debris flows are dangerous natural hazards occurring in mountainous areas throughout the world. However, they are extremely divers in composition and size, and consequently in their flow rheology. Within this study we concentrate on granular front flows in which the flowing body consists of a heavy, dense mixture of water, sediment, and boulders of different sizes, and during the flow a process of horizontal segregation leads to the development of a bouldary head.

Measurements of granular front flows at the Illgraben debris flow torrent, Swiss Alps indicate, that the vertical structure of the flow varies from a segregated layered regime close to the front to a well-mixed suspended regime in the tail of the flow. The flowing body is also influenced by check dams located within the torrent that induce vertical drops of 1.5-2m. In order to explain these phenomena accurately within the scope of a mathematical model we propose an extension to the common shallow flow theory for geophysical flows. It is capable to resolve the vertical structure of a debris flow and also accounts for dynamical effects due to strong, local changes in the topography. Higher moments of the balance laws yield additional information on the flow interior, such that we can still make use of the simplifying framework of depth-integration.

We will compare and discuss selected model properties with respect to real observations of both, large-scale natural flow events at the Swiss Illgraben torrent and meso-scale data from a new test facility in Veltheim, Switzerland that can initiate up to 60m³ of debris into a 10m wide channel.